The present invention relates to a surface-coated metallic ornamental article, especially, for personal use such as watch cases and watch straps as well as a method for the preparation thereof by providing a coating layer on the surface of a metallic substrate body. More particularly, the invention relates to a metallic ornamental article having a glassy protecting coating layer free from the disadvantageous phenomenon of blooming or whitening on the surface as well as a method for forming such a coating layer on the surface of a metallic substrate body.
It is a well known and widely practiced method of surface finishing that the surface of a metal-made personal ornamental article such as watch cases, watch straps and the like is provided with a hard protecting coating layer thereon with an object to impart the surface with insusceptibility to stain and resistance against scratches. For example, a metallic surface, either before or after a preliminary treatment such as honig treatment, hairline treatment and the like, is coated with an aqueous coating solution containing a sodium silicate and a silica sol in such a proportion as to give a SiO.sub.2 :Na.sub.2 O molar ratio of 4 to 10 followed by a heat treatment in air to form a glassy protecting layer. One of the serious disadvantages in this coating method is that the coating layer contains Na.sub.2 O which reacts with the carbon dioxide in air to form a sodium carbonate so that the phenomenon of blooming or whitening on or in the coating layer is more or less unavoidable after lapse of time. Once sodium carbonate is formed within the coating layer to cause whitening, the sodium carbonate can hardly be removed so that the aesthetic value of the ornamental article is greatly decreased.